The Friction Stir Welding (FSW) process is a solid-state based joining process, which makes it possible to weld a wide variety of materials alloys (Aluminum, Copper, Stainless Steel, etc.) to themselves and combinations (e.g. 6xxx/5xxx, 2xxx/7xxx, etc.). The joining is affected by a rotating FSW tool, which is forced into the joining area to heat it by friction and thus “plasticizes” the parts about it. Plasticized material flows around the axis of the rotating FSW tool, and the plasticized regions coalesce into sound metallurgical bonds. The process can be implemented with conventional FSW tools each consisting of a single pin and shoulder that requires backup with an anvil during welding. FIG. 1 illustrates a prior art friction stir welding tool 10 having a shank 18 that may be held in a chuck or collet of an FSW machine. Shank 18 may have a flat 19 to facilitate the application of torque to FSW tool 10.
FSW tool 10 also includes a pin 12 and shoulder 14 having a workpiece engaging surface 16. Pin 12 may include a thread 13 and flats 15. FSW tool 10 is rotated in the direction which causes thread 13 on pin 12 to push plasticized material toward the tip of pin 12. Workpiece engaging surface 16 of shoulder 14 may include a spiral thread 17. The pitch of spiral thread 17 is such that it tends to move plasticized material inwardly, toward the base of pin 12, when FSW tool 10 is rotated in the direction which tends to push plasticized material toward the tip of pin 12.
FIG. 2 illustrates two plates 111 being butt welded to each other by FSW tool 10. A backup anvil 11 on the back side of plates 111 is necessary to counteract the forging force exerted by the FSW tool onto the plasticized joint and prevent escape of plasticized material, and produce a smooth surface on the back side. Hence, FSW tools similar to FSW tool 10 have the limitation that they cannot be employed for welds for which it is not possible to access the back side of the components being welded.
In order to weld components wherein it is not possible to access the back side of the weld to place a backup anvil, bobbin-type tools may be employed. Such tools include two shoulders and a pin between them. The concept for such tools was patented by Kevin Colligan on 2003 Dec. 30, U.S. Pat. No. 6,660,075 (FIG. 3). The bobbin-type FSW tool 20 illustrated in FIG. 3 includes a FSW pin 21 and a pair of shoulders 22, shoulders 22 including workpiece engaging surfaces 23. Since the shoulders 22 have the taper angle 24, they can be integral with pin 21. In order to impart the forging force to weld workpieces 111 having some tolerance in thickness, the workpiece engaging surfaces 23 are tapered away from workpieces 111 at the taper angle 24 shown in FIG. 3.
Not only does the taper angle 24 enable workpieces having somewhat variable thicknesses to be welded, it also ensures that the necessary forging force is applied to the plasticized region whereby plasticized material is confined to the weld region, and smooth surfaces are produced on the upper and lower surfaces of the weld. The teachings of U.S. Pat. No. 6,660,075 are included herein by reference thereto.
A more complete drawing of a bobbin-type FSW tool is given in FIG. 4. Bobbin-type FSW tool 30 includes a shank 36 and an FSW pin 39. Pin 39 includes a proximal pin portion 31 on the proximal side of the center 38 of pin 39, and a distal pin portion 37 on the distal side of the center 38 of pin 39. Proximal pin portion 31 and distal pin portion 37 have opposite pitch, and FSW tool 30 is rotated in the direction which tends to cause plasticized material to flow towards the center 38 of pin 39.
FSW tool 30 also includes a proximal shoulder 32 having workpiece engaging surface 33 and distal shoulder 34 having workpiece engaging surface 35. Again, the workpiece engaging surfaces 33 and 35 are tapered to tolerate variations in workpiece thickness and to apply the required forging force to the plasticized material. The bobbin-type FSW tool 30 is described in the copending patent application entitled “Advanced Friction Stir Welding Tools”, application Ser. No. 11/100,878 Filed on Apr. 7, 2005.
FSW tool 30 includes a tension member 27, which is placed in tension by nut 28 acting through spring washer 29. The purpose of tension member 27 is to place pin 39 in compression to prevent fracture of pin 39 due to the combination of severe cyclic torsion and bending moments it experiences during friction stir welding.
FIG. 5 illustrates the bobbin type FSW tool 30 in position for welding joint 113, which is one of a pair of joints 113 and 114 needed to produce a rectangular tube from a pair of elongate members, each elongate member having a cross-section shaped like a square bracket, each elongate member corresponding to one half of the cross-section of the rectangular tube. It is noted that bobbin tools of the type taught by Mr. Colligan are capable of welding only one joint at a time.
FIG. 6 illustrates a prior art FSW tool 50 having superior mechanical properties. It includes an integral shank-pin ensemble with a shoulder 54 threaded onto the shank-pin ensemble. FSW tool 50, preferably, has a close fit 57 between the shank 53 and the inside of the shoulder 54. It also has a close fit 58 between the pin 52 and the inside of shoulder 54 near the base of pin 52, and it has a firm stop 59 between the inside of shoulder 54 and the shank 53. FSW tool 50 is presented in the copending patent application: “Advanced Friction Stir Welding Tools”, application Ser. No. 11/100,878 Filed on Apr. 7, 2005.
That application also advances the concept of including an internal tension member to provide compression loading of the pin of a bobbin type FSW tool. FIG. 7 provides preferred internal detail regarding the prior art bobbin-type FSW tool. Preferably, FSW tool 30 includes a snug fit 42 at the proximal end of proximal shoulder 32, snug fit 41 at the distal end of proximal shoulder 32, and firm stop 42. Likewise, FSW tool 30 includes snug fit 44 at the proximal end of distal shoulder 34, and the firm stop 45. Both shoulder 32 and shoulder 34 may be assembled by threading them on from the distal end of FSW tool 30.
While the FSW tools described above have a number of desirable features, each is capable of welding only one joint at a time. A need remains for a FSW tool which can make a plurality of welds such as joint 113 and 114 shown in FIG. 5.